Such semi-finished steel products are used in the shipbuilding industry and for other large-scale structures such as oil production platforms and include steel plates, for example of thickness 6 to 75 mm, bars, girders, and various steel sections used as stiffening members. The most important heat-intensive process is welding; substantially all such semi-finished steel products are welded. Other important heat-intensive processes are cutting, for example oxy-fuel cutting, plasma cutting or laser cutting, and heat fairing, where the steel is bent into shape while being heated. These steel products are often exposed to the weather during storage before construction and during construction, and they are generally coated with a coating called a “shop primer” or “pre-construction coating” to avoid corrosion of the steel occurring before the steel construction, e.g. a ship, is given its full coating of anticorrosive paint, thereby avoiding the problem of having to overcoat or remove steel corrosion products. In most big shipyards, the shop primer is applied as one of several treatments carried out on a production line in which the steel is for example preheated, shot- or grit-blasted to remove mill scale and corrosion products, shop primed, and passed through a drying booth. Alternatively, the shop primer can be applied by a trade coater or steel supplier before the steel is delivered to the shipyard or other construction site.
Although the main purpose of the shop primer is to provide temporary corrosion protection during construction, it is preferred by shipbuilders that the shop primer does not need to be removed but can remain on the steel during and after fabrication. Steel coated with the shop primer thus needs to be weldable without removal of shop primer and to be overcoatable with the types of protective anti-corrosive coatings generally used on ships and other steel constructions, with good adhesion between the primer and the subsequently applied coating. The shop primed steel should preferably be weldable without any significant detrimental effect on the quality of the weld or on the speed of the welding process and should be sufficiently resistant to heat for the shop primer to retain its anticorrosive properties in areas heated during fairing or during welding of the opposite face of the steel.
Commercially successful shop primers available today are solvent borne coatings based on prehydrolyzed tetraethyl orthosilicate binders and zinc powder. Such coatings contain a large proportion of volatile organic solvent, typically about 650 grams per liter, to stabilize the paint binder and to enable the product to be applied as a thin film, typically of about 20 microns thick. Release of volatile organic solvent can be harmful to the environment and is regulated by legislation in many countries. There is a need for a shop primer which releases no, or much less, volatile organic solvent. Examples of such coatings are described in U.S. Pat. No. 888,056 and JP-A-7-70476.
JP-A-6-200188 is concerned with shop primer coatings and mentions the possibility of using an aqueous alkali silicate salt type binder. Coatings comprising an aqueous alkali metal silicate and zinc powder are also proposed in GB-A-1226360, GB-A-1007481, GB-A-997094, U.S. Pat. No. 230,496, and JP-A-55-106271. Alkali silicate binders for anticorrosive coatings are also mentioned in U.S. Pat. Nos. 3,522,066, 3,620,784, 162,169, and 479,824.
KR 8101300 describes a process in which a composition comprising an alkyl orthosilicate or an alkali metal silicate, zinc powder, and aluminium oxide powder or titanium oxide powder is applied to a steel plate and subsequently left to dry at 20° C. and 75% RH. The document does not mention or suggest any advantageous effect of these specific drying conditions on the film properties nor on the rate of film property development.
We have found that primer coatings based on an aqueous alkali silicate binder containing zinc powder can give adequate corrosion protection and allow the steel surfaces they cover to be welded, but give rise to problems when overcoated. The aqueous silicates contain a large quantity of alkali metal cations, which are required to keep the silicate in aqueous solution and these ions are still present in the coating after it has dried. We have found that, if primer coatings having these large quantities of alkali metal ions are overcoated with any conventional organic coating and then immersed in water, blistering (local delamination of the coating) occurs. We have performed tests which show that this problem can be reduced if the coating is weathered outside for some time after application of the shop primer or washed prior to overcoating. However, these processes are not compatible with use in today's high productivity shipyards.
Aqueous silica sols having a very low alkali metal ion content are available commercially, but coatings based on such sols normally have very poor (initial) film strength in terms of adhesion, cohesion, hardness, and resistance to abrasion and water. These poor physical properties of the coating make it susceptible to damage during handling or further processing. This brings the potential requirement of significant coating repair with major cost implications. Suggested improvements to silica sol coatings are described in U.S. Pat. No. 3,320,082, which adds a water-immiscible organic amine, GB-A-1541022, which adds a water-soluble acrylamide polymer, and GB-A-1485169, which adds a quaternary ammonium or alkali metal silicate, but such coatings have not achieved physical properties similar to those of coatings based on alkali metal silicates. Coatings based on silica sols show low levels of blistering when overcoated/immersed. Although the water-soluble salt content and osmotic pressure are low, blistering can still occur, as the coating exhibits little resistance to blister initiation/growth due to its poor physical properties.
There is a need for a water based shop primer of low alkali metal ion content which has improved adhesion to substrates and improved film strength in terms of the properties discussed above to resist blister initiation and growth. Further, there is a need for a blister-free water based shop primer showing fast development of the physical properties of the coating after application of the shop primer to enable handling and further processing of the substrate without the risk of damaging the coating.